1. Field of Invention
The present invention relates, in general, to a circuit for precisely detecting the error information of an output voltage from the winding voltage of a transformer, including a ringing voltage attributable to leakage inductance, in a Switching Mode Power Supply (SMPS), and a circuit and method for feeding the error information of the output voltage back to a control circuit, and, more particularly, to an SMPS, which can be implemented based on precise primary regulation using the invention. Further, the present invention also relates to a flyback converter, a buck-boost converter, and a buck converter, each including the error feedback circuit and method for the SMPS.
2. Description of Related Art
Generally, a Switching Mode Power Supply (SMPS) regulates an output voltage using a secondary regulation method. Such a secondary regulation method is implemented such that a typical error detection circuit is provided on an output voltage drawing part to precisely detect an output voltage, and such that detected error information is fed back to a power conversion unit through a photocoupler or the like, thus enabling the output voltage to be regulated. Since the secondary regulation method having such a structure is well known to those skilled in the art, a detailed description thereof is omitted.
An output voltage regulation circuit based on the above-described secondary regulation has high voltage stability, but increases the costs of a power circuit because expensive parts are used and the structure of the circuits is complicated.
Accordingly, when highly stable output voltage is not required, there are many factors to consider in order to reduce the costs of power circuits.
FIG. 1 illustrates a primary regulation-based power circuit generally proposed as an inexpensive power circuit.
This circuit detects voltage induced to an auxiliary winding T1-3, rather than the secondary side of a transformer T1, and feeds the detected voltage back to a control circuit, thus ultimately regulating an output voltage, which is output through a secondary winding T1-2. This method is called a primary regulation method.
The operation of the power circuit of FIG. 1 is described.
During the conduction period of a switching element U2, magnetic energy is stored in the primary winding T1-1 of the transformer T1, and during the non-conduction period of the switching element U2, the magnetic energy stored in the primary winding T1-1 is transferred to the secondary winding T1-2, closely coupled thereto, is rectified by a diode D2, is accumulated in a condenser C3, and is then supplied to a load (not shown).
Meanwhile, the primary winding T1-1 of the transformer T1 is wound to be closely coupled to the secondary winding T1-2, but, in practice, leakage inductance, which is not coupled to the secondary winding T1-2, exists. The energy stored in the leakage inductance is not transferred to the secondary winding T1-2, and extinguishes itself while undergoing resonance together with stray capacitance that exists in the primary winding T1-1. A clamp circuit 13 is typically used to suppress the magnitude of a surge spike voltage attributable to leakage inductance.
Therefore, the voltage induced to the auxiliary winding T1-3 of the transformer T1 appears in such a way that the output voltage component of the secondary winding T1-2, obtained according to the turns ratio, and a spike voltage component, generated due to leakage inductance, are simultaneously induced.
The voltage induced to the auxiliary winding T1-3 is fed back to a control circuit 11 through an error amplifier 12 and a feedback condenser Cfb1.
FIG. 2 is a diagram showing an example of the waveform of the voltage induced to the auxiliary winding T1-3.
In FIG. 2, the period from T20 to T21 is the conduction period of the switching element U2 of FIG. 1. During this period, an input voltage V1 is applied to the primary winding T1-1, and a ‘negative’ voltage proportional to the turns ratio of the primary winding T1-1 to the auxiliary winding T1-3 is applied to the auxiliary winding T1-3. At time point T21, the switching element U2 is turned off, and the winding voltage is reversed at this time and reaches a voltage Vpeak, which is much higher than an output voltage component, because a surge voltage is generated due to the influence of leakage inductance. The peak value Vpeak of the surge voltage is rectified by the diode D3 of FIG. 1, is charged in the condenser C4, and is used as a feedback value required for voltage control.
Therefore, the voltage charged in the condenser C4 contains an output voltage Vo that is output to the secondary winding T1-2 and a surge voltage component attributable to leakage inductance.
The voltage induced to the auxiliary winding T1-3 of FIG. 2 exhibits different waveforms under a heavy load and a light load. FIG. 2(a) is a waveform diagram when the load is heavy, that is, under a heavy load, and FIG. 2(b) is a waveform diagram when a load is light, that is, under a light load.
Under the heavy load, as shown in FIG. 2(a), since the amount of energy stored in the primary winding T1-1 of the transformer T1 during the conduction period of the switching element U2, is large, high surge voltage attributable leakage inductance is generated. Under the light load, as shown in FIG. 2(b), since the amount of energy stored in the primary winding T1-1 of the transformer T1 is small, low surge voltage attributable to leakage inductance is generated.
That is, the surge voltage attributable to leakage inductance is changed according to the lightness or heaviness of the load. When control is performed such that the voltage charged in the condenser C4 is maintained at a constant voltage, output voltage inevitably varies. Consequently, under the light load, output voltage increases, and, under the heavy load, output voltage decreases.
When the output voltage is controlled using a typical primary regulation method, the output voltage under no load is increased by an amount ranging from 30 up to 100% of the output voltage under the maximum load. That is, when the output voltage at the maximum load is 5V, the output voltage is increased from 6.5 to 10V under no load.
In summary, the prior art secondary regulation method has high stability, but is problematic in that it is expensive, and the primary regulation method is inexpensive, but is problematic in that the range of variation between voltage at the maximum load and voltage at no load is wide, thus deteriorating the stability of output voltage.